1. Field of the Invention
The present invention relates to the implementation of a circuit for simulating the electrical characteristics of a so-called "break-over" component. A break-over component, for example a one- or two-way Shockley diode, a thyristor or a triac, is characterized by a discontinuity in its current-voltage characteristic.
2. Discussion of the Related Art
FIGS. 1 and 2 show, respectively, an equivalent circuit of a Shockley diode and its current-voltage characteristic.
A one-way Shockley diode can be broken up, functionally, into a thyristor Th associated with a Zener diode DZ. The Zener diode connects the gate of the thyristor to its anode.
FIG. 2 illustrates the current-voltage characteristic of a Shockley diode. This characteristic shows the relationship between the current I running through the circuit of FIG. 1 as a function of the voltage V.sub.KA between the cathode terminal K and the anode terminal A. It is assumed that the thyristor is initially blocked (non-conducting) and that voltage V.sub.KA starts increasing from a zero value. As long as voltage V.sub.KA has not reached the avalanche voltage V.sub.B R of Zener diode DZ, current I is zero (neglecting any leakage). Once voltage V.sub.BR is reached, the current increases linearly until thyristor Th triggers. This triggering happens at a so-called "break-over" operating point, for a voltage Vbo and a current Ibo. From this point, thyristor Th is on and voltage V.sub.KA across it becomes very low. The slope of the current variation in this operating range corresponds to the voltage drop in the thyristor in the on-state.
Once current I reaches, as it decreases, an on-state holding current Ih of the thyristor, the thyristor becomes non-conducting. This new non-conduction of the thyristor corresponds to the zero current portion of the characteristic.
A current-voltage characteristic of a thyristor or of a triac in its positive voltage and current quadrant is substantially similar to that of a Shockley diode.
A difference is that, for a thyristor or a triac to trigger, the gate current must further be higher than a given threshold Igt. Conventionally, the locking current is generally designated with symbol Il.
A problem appears when a break-over component is desired to be tested. In particular, when it is desired to determine the characteristic parameters (Ih, Il, Igt, Ibo and Vbo and other) of a new type of break-over component, a tester is used to measure the different triggering thresholds. The tester includes several current and voltage sources for supplying the component to be tested. In the case of a break-over component test, the tester is generally driven by a computer program determining a testing wave. For a Shockley diode, such a wave includes a period during which the voltage increases to detect a current variation and, thus, voltage V.sub.BR, the break-over point Ibo-Vbo and other characteristic predetermined operating points. After, the wave decreases to cause a decrease of the current through the Shockley diode, in order to detect the holding current Ih. The current and voltage peaks of the testing wave may reach values of several hundreds of amperes and volts. The accuracy of the current and of the voltage provided by the testing device may undergo variations, as well as there may be areas of discontinuity of the current or voltage. This occurs, for example, upon a changing of range corresponding to a switching between different voltage and/or current sources of the tester.
Resistors are generally used to calibrate the current and voltage sources as well as the measuring circuits of the tester. If such a calibration method effectively ensures that the voltage and current sources can operate without discontinuities with a linear relation, it does not enable checking of the operation of the tester in the mode which will be used to test a break-over component. Indeed, by means of a resistive load, the operation of the sensor cannot be checked during the switchings which are necessary in the operation of a break-over component. Further, the programs used to produce a linear current or voltage are not the same as those which condition the shape of the testing wave of a break-over component. To be properly checked, a tester has to be operated in the same conditions as on the component to be tested.
However, it is not possible to use a standard break-over component to check the operation of the tester. Indeed, the holding current Ih varies from one component to another and during operation, especially, according to the testing wave which is applied across the component. This is actually the reason why the characteristic parameters of a Shockley diode or of a triac are given for a determined testing wave and as maximum and minimum values for a given type of component.
Besides, a multitude of standard components should be available to check the operation of the tester in an entire range of currents and voltages.